Nanograin Evolution in Cold Crystallization of Syndiotactic Polystyrene As Illustrated via in-Situ Small/Wide-Angle X-ray Scattering and Differential Scanning Calorimetry

Abstract

Structural evolution of syndiotactic polystyrene (sPS) in a cold crystallization process was quantitatively examined with in-situ small/wide-angle X-ray scattering and differential scanning calorimetry (SAXS/WAXS/DSC). After removal of background scattering from fractal-like matrix structure, SAXS profiles obtained during programmed heating of an amorphous sPS specimen from 30 to 240 at 10°C/min can be interpreted with a similar sequence of events previously observed in cold crystallization of poly(9,9-di-noctyl-2,7-fluorene) (PFO). Specifically, the nanograin evolution of sPS involves four stages: (1) the frozen-in stage below 90°C, (2) the nucleation of oblate-like nanograins with a constant radius of gyration R g ≈2.6 nm between 90 and 130°C, (3) the growth of nanograin size to R g ≈ 3.2 nm concomitant with the emergence and development of WAXS-determined crystallinity (X c,WAXS ) from 130 to 180°C, and finally (4) the coalescence (and thickening) of the nanocrsytals into a greater size of R g ≈ 4.6 nm upon further heating up to 240°C. Developments in the DSC-determined crystallinity (X c,DSC ) coincided with nucleation and growth stages, whereas the SAXS-determined heterogeneity (Q inv ) increased steadily throughout nucleation, growth, and coalescence stages. Little changes of morphological features in the nanometer-length scale can be observed with subsequent isothermal annealing at 240°C up to 1 h; the final size of coalesced nanograins at this temperature is therefore attributed to the balance between the tendency to eliminate lateral surface via coalescence and the opposing strain field (due to locked and tightened entanglements) in the surrounding matrix. Delicate differences in nanograin size and shape during cold crystallization processes of sPS and PFO are discussed in terms of differences in chain rigidity (random coiled vs semirigid) and melt structure (isotropic vs nematic).

Effect of Film Formation Method and Annealing on Crystallinity of Poly(L-Lactic Acid) Films

“…For amorphous material, such as the non-annealed spin coated film, crystallization from glassy state can generate crystal structures different from category 1 spherulites. One common feature is the nodule structure [10][11][12], the nanometer-sized crystalline particles developed during annealing from the glassy state [13]. As annealing time and/or temperature increases, nodules enlarge, and may develop into larger rod-like crystals [10].…”

Effect of Film Formation Method and Annealing on Morphology and Crystal Structure of Poly(L-Lactic Acid) Films

“…Crystallization from the melt follows a different route, in which nanometer-sized nodules are developed first followed by coalescing with each other to form larger crystals such as lamellae [9]. The concurrent orientation adjustment of neighboring nodules is required for prefect coalescence.…”

“…The resulting structure is composed of nanometer-sized nodules in a planar assembly. The nodules coalesce to develop lamellae [9], while fringed micellar crystallization can also occur simultaneously with the folding structure, which roughens the fold surface [37]. The sketch of the lamellar structures in the annealed spin coated film is given in Fig.…”

“…As annealing time and/or temperature increase, the nodules enlarge, and may develop into larger needle-like crystals [8]. It is also observed that nodules develop into lath-like lamellae through merging or lateral aggregation [9,10]. At larger scale, the sheaf-like structures can be observed [12,13], and category 2 spherulites, in which lamellae grow unidirectionally in the central region and the spherical symmetry does not extend to the center [6], are developed [14].…”

“…Annealing such films from the glassy state can generates crystal structures different from those developed in as cast films. One common feature is the nodular structure [8][9][10]; the nodule is the nanometer-sized crystalline particle developed during annealing from the glassy state [11]. As annealing time and/or temperature increase, the nodules enlarge, and may develop into larger needle-like crystals [8].…”